The invention relates to a sprung steering system front structure for bicycles having a
shaft tube with an upper end and PA1 a front structure shaft mounted so as to rotate about a rotation axis at the upper end of the shaft tube, PA1 with a receiving ring for the handle-bar, PA1 a shock absorbing spring mechanism being provided between the shaft tube and the front structure shaft. PA1 the spring mechanism is a rubber torsion spring with a rubber element, which is vulcanized in between an outer metal casing and an inner metal sleeve, PA1 the shaft tube has at the upper end a fork, in which is fixed the spring mechanism by means of a screw passing through the fork legs, so that the inner metal sleeve is fixed to the fork and the outer metal casing to the front structure shaft, PA1 the inner faces of the fork legs and the front faces of the inner metal sleeve are serrated for a reciprocal engagement and form a tooth system, PA1 on the outer metal casing is provided a stop web for a setscrew, PA1 which is located oh the fork, PA1 the outer metal casing and the inner metal sleeve are elastically rotatable against one another about an angle for shock absorption in the case of a force action under torsional stressing of the rubber element, PA1 so that following the loosening of the screw a varyingly high pretension can be set by swinging up the front structure shaft with an angular rotation of the tooth system, in which PA1 after retightening the screw and forcing back the front structure shaft against the spring tension of the rotating rubber element, the setscrew is screwed against the stop web of the metal casing and in this way the desired pretension is stored, PA1 with an increase in the pretension the spring tension of the steering system front structure correspondingly increases and PA1 by loosening or tightening the setscrew the angle between the shaft tube and the front structure shaft can be individually varied.
In order to avoid the transmission of impacts or shocks acting on the front wheel to the steering system numerous different spring and shock absorbing systems are already known, such as e.g. telescopic spring systems, but they are fault-prone and in particular not maintenance-free, whilst being expensive to manufacture and fit.
A spring steering system front structure of the aforementioned type can e.g. be gathered from the Journal RADFAHREN 4/94. In said known steering system front structure below the pivot bearing of the front structure shaft and namely on the shaft tube there is a spring element comprising a PU-ring acting as a rigid elastomer buffer. Impacts transferred to the shaft tube are absorbed by compression on the rigid elastomer buffer, on which is supported a pressure piece fixed to the lower end of the front structure shaft. By means of elastomeric PU-rings as the buffer element only a vibration damping, but not a spring suspension can be obtained. Thus, the maximum spring deflection, when only using elastomer elements with a very low degree of hardness, is 1.8 cm. Therefore the known spring steering system front structure with a soft elastomer element can be used for damping vibrations, whose transmission from the frame via the shaft tube to the front structure shaft is to be avoided. However, with the known means a spring suspension cannot be obtained.
The problem is therefore to provide a spring steering system front structure of the aforementioned type permitting the obtaining of an effective spring cushioning of road impacts and which also allows an easy adjustment of the spring tension.